Attenuation of Cytomegalovirus Virulence

ABSTRACT

A method is provided for attenuating a cytomegalovirus comprising functionally disrupting an open reading frame of a Toledo genome region or its homolog and making chimeric CMV virus genomes.

REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 09/724,935, filed on Nov. 28, 2000, now U.S. Pat. No. 7,204,990, wich is hereby incorporated by reference herein in it entirety for all purposes.

TECHNICAL FIELD

The present invention is related generally to methods and compositions for treating or preventing cytomegalovirus (CMV) infections, such as congenital CMV disease, CMV retinitis, CMV mononucleosis, and the like, and methods of attenuating pathogenic cytomegalovirus isolates and strains, genetically engineered cytomegaloviruses and combinations thereof, methods for altering the phenotype of CMV viruses, attentuated viral vaccine compositions, and uses thereof. More particularly, the present invention is related to methods and compositions for prophylaxis and therapy of human cytomegalovirus infection, including the use of methods that functionally inactivate a subset of cytomegalovirus genes present in pathogenic isolates of human cytomegalovirus.

BACKGROUND

Cytomegalovirus (CMV) is a widespread herpesvirus in the human population, with between 0.2 and 2.2% of the infant population becoming infected in utero and another 8-60% becoming infected during the first six months of life (Reynolds et al. (1973) New Engl. J. Med. 289: 1). Although CMV infections are most commonly subclinical, CMV-induced sensorineural hearing loss and fatal cytomegalovirus infections (“cytomegalic inclusion disease”) are important public health problems. Moreover, CMV is one of the more common opportunistic infections associated with Acquired Immune Deficiency Syndrome (“AIDS”) and frequently produces disease, with recurrent infection occurring in HIV-positive individuals, typically taking the form of retinitis or ulcerative lesions in the colon and esophagus, and occasionally producing extensive necrotization of the bowel with a grave prognosis (Rene et al. (1988) Dig. Dis. Sci. 33: 741; Meiselman et al. (1985) Gastroenterology: 88: 171). Cytomegalovirus (CMV) infection is the major infectious cause of mental retardation and congenital deafness. CMV is also responsible for a great deal of disease among the immunosuppressed, producing general and often severe systemic effects in patients with AIDS, in organ transplant recipients who have been iatrogenically immunosuppressed, and in bone marrow transplant patients.

It is clear that cytomegalovirus infections are a significant human health problem. Therefore, it is desirable to develop prophylactic and therapeutic methods and compositions to prevent cytomegalovirus infection and/or inhibit recurrent infectious outbreaks from persistent latent infections, particularly for treating CMV retinitis, CMV mononucleosis, and related CMV pathology in human patients.

One approach that has been used to treat herpesvirus infections is to inhibit CMV viral DNA replication. For example, viral DNA replication can frequently be inhibited by agents that inhibit virally-encoded DNA polymerase. The most notable examples of such inhibitors of viral DNA polymerase are acyclovir, ganciclovir, citrusine-I, and the acyclic guanosine phosphonate (R,S)-HPMPC (Terry et al. (1988) Antiviral Res. 10: 235; Yamamoto et al. (1989) Antiviral Res. 12: 21). However, these compounds are not completely selective for viral thymidylate synthetases or DNA polymerases and therefore can disadvantageously cause inhibition of host DNA replication at high doses. Moreover, the development of mutant viruses which are resistant to the inhibitory effects of these compounds have been reported, and appear to result from mutations in the viral DNA polymerase (Coen et al. (1982) J. Virol. 41: 909; Coen et al. (1980) Proc. Natl. Acad. Sci. (U.S.A.) 77: 2265; Larder et al. (1987) EMBO J. 6: 169). Thus, while CMV infections, such as CMV retinitis, can be initially treated with foscarnet and ganciclovir, after a period of time CMV replication and progression of the pathological viral infection recurs.

Passive immunization with antibodies (e.g., immune globulin) has been tested in combination with ganciclovir for therapeutic efficacy in humans. Such antibody preparations are obtained from the serum of donors, who possess a high antibody titre to the virus as a result of an earlier infection. One disadvantage of such conventional antibody preparations is the limited number of suitable donors and the poor reproducibility or quality of the various preparations, including potential contamination with pathogens and pathogenic viruses. Unfortunately, the use of intravenous immune globulin in combination with ganciclovir apparently does not produce significantly improved efficacy as compared to ganciclovir treatment alone (Jacobson et al. (1990) Antimicrob. Agents and Chemother. 34: 176).

The safety and pharmacokinetic profiles of anti-cytomegalovirus monoclonal antibodies are discussed in Aulitzky et al. (1991) J. Infect. Dis. 163: 1344 and Drobyski et al. (1991) Transplantation 51: 1190. However, none of the reported human anti-CMV monoclonal antibodies have been shown to possess significant therapeutic efficacy in treating CMV infections (e.g., retinitis) in humans.

Attempts to use recombinantly produced hCMV glycoproteins as a subunit vaccine to provide protective immunity against hCMV infection and pathogenesis have not proven to be effective, but remain candidates for additional evaluation.

Thus, there exists a need in the art for effective methods and compositions for inhibiting human cytomegalovirus replication, attenuating CMV virulence in vivo, neutralizing CMV virions, and for preventing and treating human cytomegalovirus infections, and especially CMV infections in preborns, newborns, and immunosuppressed patients such as AIDS patients. For example but not limitation, a suitable attenuated human CMV vaccine which elicits satisfactory immunoprotection against CMV infection is needed in the art. The present invention fulfills these and other needs.

SUMMARY OF THE INVENTION

A basis of the present invention is the surprising and unexpected finding that: (1) clinical isolates of pathogenic CMV variants contain a genomic region (“virulence region”) which typically is not present in CMV strains which have undergone extensive laboratory passaging of the virus in cell culture (hereafter termed “highly passaged strain variants”) and (2) functional disruption (e.g., deletion or insertional inactivation and the like) of genes in this genomic region produces a substantial attenuation of CMV virulence and/or pathogenicity in vivo. Furthermore, the virulence region of a clinical isolate of CMV is frequently deleted, rearranged, or substantially changed over the course of passaging the virus in cell culture.

In one aspect of the invention, the virulence region is obtained from an early passage Toledo strain and is conveniently termed the “Toledo genomic region” herein, although equivalent (e.g., homologous) regions or subsequences thereof are present in other clinical isolates of CMV besides the Toledo strain of CMV; the term “Toledo genomic region” encompasses these homologous regions in other clinical CMV isolates, many early passage CMV strains, and non-isolated pathogenic CMV variants.

The Toledo genomic region which is present in pathogenic CMV isolates and which is typically substantially absent in highly passaged CMV strains (e.g., AD169, high-passage Towne) has been sequenced and several open-reading frames have been identified (PCT Publication WO96/30387, U.S. Ser. No. 08/414,926, U.S. Ser. No. 08/644,543 filed 10 May 1996, each incorporated herein in their entirety by reference). Functional disruption of these open reading frames, either singly or in combination, has been unexpectedly found to substantially reduce virulence of the resultant CMV mutant(s) in vivo. Thus, in part, the invention provides methods and compositions for suppressing or inactivating expression of genes of the Toledo genomic region and its homolog regions in other CMV variants, and thereby reducing virulence and pathogenicity of clinically important CMV variants to generate a “Toledo region-attenuated CMV variant”; such Toledo region-attenuated CMV variants have altered phenotypes which generally make them candidates for use in live attenuated virus vaccines for prophylaxis and/or treatment of CMV disease. The invention is, in part, further based on the heretofore unrecognized finding that pathogenic clinical isolates of CMV have a distinct genome as compared to the commonly used laboratory-passaged strains of human CMV (e.g., AD169, highly-passaged Towne), and that the genomic region which is present in the clinical isolates and which is substantially absent in laboratory-passaged strains confers enhanced virulence in vivo. Most common approaches to development of CMV therapies and vaccines have heretofore relied on laboratory-passaged strains which typically lack all or part of the Toledo genomic region and the genes encoded therein which have been unexpectedly found to confer enhanced in vivo virulence and are believed to contribute to clinical pathology and CMV-related disease.

The invention provides a method for attenuating virulence of CMV comprising functionally inactivating at least one open reading frame in a virulence region of a CMV genome having substantial identity to at least 300 bp, typically at least 500 bp, of a 15 kb sequence present in the genome of the Toledo strain of CMV and absent from the genome of the AD169 strain of CMV and/or absent from the genome of highly-passaged Towne (i.e., more than 50-100 passages). In an aspect, the method functionally inactivates at least one open reading frame present in a genomic region of a CMV genome having substantial identity to at least 300 bp of a 13 kb sequence present in the genome of the Toledo strain of CMV and absent from the genome of the Towne strain of CMV. In an embodiment, the method functionally inactivates at least one open reading frame present in a genomic region of a CMV genome having substantial-identity to at least 500 bp of the sequence shown in FIGS. 1A through 1R (SEQ ID NO: 1). In an embodiment, the method functionally inactivates at least the open reading frame corresponding to UL148 as identified herein. In a variation, the method functionally inactivates open reading frames in the region spanning UL138 to UL14.8. In an embodiment, the method functionally inactivates UL138, UL139, UL140, UL141, UL142, UL143, UL144, UL145, UL146, UL147, and/or UL148. In a variation, UL148 is inactivated singly or in combination with other open reading frames of the Toledo genomic region. In a specific embodiment, UL148 is inactivated in combination with UL141 and/or UL144. Typically, such Toledo region-attenuated CMV variants comprise at least 500 bp of the Toledo genomic region or a homolog region having at least 80 percent sequence identity; frequently they comprise at least 1.0 kbp of the Toledo genomic region or homolog virulence region; often they contain at least 5.0 kbp to 8.0 kbp of the Toledo genomic region or homolog virulence region, and can comprise up to a complete Toledo genomic region or homolog virulence region. It is possible for a synthetic virulence region to be comprised of portions of two or more virulence regions (e.g., such as a chimeric virulence region comprising part of the Toledo genomic region from a first clinical isolate with a complementing portion of the Toledo genomic region of a second clinical isolate).

In an aspect, the invention provides a method for attenuating a CMV strain or isolate containing an encoding polynucleotide sequence encoding a polypeptide which is at least 80 percent sequence identical to a polypeptide encoded by UL138, UL139, UL140, UL141, UL142, UL143, UL144, UL145, UL146, UL147, and/or UL148 of the Toledo genomic region; the method comprising functionally inactivating (e.g., deleting or introducing a nonsense or missense mutation) said encoding polynucleotide sequence to produce a Toledo region-attenuated CMV variant. In a variation, all open reading frames (ORFs) in the CMV isolate that are at least 80% sequence identical to the corresponding sequence of the Toledo genomic region are functionally inactivated. In a variation, all open reading frames (ORFs) in the CMV isolate that are at least 80% sequence identical to UL138, UL139, UL140, UL141, UL142, UL143, UL144, UL145, UL146, UL147, and/or UL148 of the Toledo genomic region are functionally inactivated. In an alternate variation, only one or a subset of the open reading frames (ORFs) in the CMV isolate that are at least 80% sequence identical to the corresponding sequence(s) of the Toledo genomic region are functionally inactivated. Such Toledo'region-attenuated CMV variants comprise at least 500 bp of a Toledo genomic region and can comprise up to a complete Toledo genomic region (including a chimeric Toledo genomic region composed from distinct clinical isolates or strains).

In an aspect, the invention provides a recombinant CMV virus, comprising a genome having at least 500 bp of a virulence region wherein at least one ORF has been functionally inactivated by a genetic alteration which is predetermined and/or which does not occur in known isolates or strains of CMV regardless of passage history.

In an aspect, the method of attenuating virulence comprises functional inactivation of open reading frames by predetermined structural mutation (e.g., deletion, insertion, missense or nonsense mutation, and the like) of at least one open reading frame, or a predetermined mutation of a transcriptional control sequence that controls transcription of the open reading frame, or predetermined mutation of a splicing signal sequence or the like necessary for efficient expression of the encoded gene product of the open reading frame. In an embodiment, a selectable marker gene is introduced into an open reading frame, often in the portion of the open reading frame believed to encode the amino-terminal two-thirds of the gene product, to structurally disrupt the open reading frame and result in the inactivation of the open reading frame's capacity to encode its functional gene product. In a variation, open reading frame UL148 is structurally disrupted by mutation; in one embodiment the structural disruption results from insertion of a selectable and/or screenable marker gene (e.g., gpt/lacZ). In an embodiment, a selectable marker gene is used to replace all or part of at least one open reading frame, such as by replacement of a deleted region of the Toledo genomic region with a selectable marker gene. In a variation, a region spanning open reading frame UL138 to UL148 is structurally disrupted by mutation; in one embodiment the structural disruption results from deletion of the UL138-UL148 region and replacement with a selectable and/or screenable marker gene (e.g., gpt/lacZ).

In an aspect, the functional inactivation of a Toledo genomic region gene is provided by transcriptional and/or translational suppression with an antisense polynucleotide having a sequence of at least 15 nucleotides, typically at least 25 nucleotides, that are substantially complementary to a Toledo genomic region, most usually the antisense polynucleotide is substantially complementary to an open reading frame sequence of a Toledo genomic region open reading frame. In an embodiment, the antisense polynucleotide is substantially complementary to at least 25 nucleotides of UL148. In an embodiment, the antisense polynucleotide is complementary to UL148 and further comprises additional 5′ and/or 3′ nucleotide(s) which are not substantially complementary to UL148. In variations, the antisense polynucleotides comprise non-natural chemical modifications, and can include, for instance, methylphosphonates, phosphorothioates, phosphoramidites, phosphorodithioates, phosphorotriesters, and boranophosphates. In a variation the antisense molecules can comprise non-phosphodiester polynucleotide analogs wherein the phosphodiester backbone is replaced by a structural mimic linkage include: alkanes, ethers, thioethers, amines, ketones, formacetals, thioformacetals, amides, carbamates, ureas, hydroxylamines, sulfamates, sulfamides, sulfones, and glycinylamides. In a variation, the invention provides peptide nucleic acids (PNAs) having a U nucleobase sequence which is substantially complementary to a Toledo genomic region sequence, such as an open reading frame W (e.g., UL148, UL141, UL142, etc.).

The invention also provides attenuated live virus CMV vaccines wherein at least one open reading frame of a Toledo genomic region is structurally disrupted. Typically, the UL148 open reading frame is structurally disrupted, either singly or in combination with other Toledo region open reading frames (e.g., UL141, UL144, and the like). Often the disruption of the open reading frame is an insertion, deletion, or replacement mutation which confers the property of reduced virulence as determined by a suitable in vivo virulence assay (e.g., see Experimental Examples). Toledo genomic region mutants which exhibit at least one log reduction, preferably two logs or more reduction, in virulence as determined by in vivo virulence assay, or other equivalent virulence measure, are attenuated CMV vaccines. Such attenuated CMV vaccines are used to immunize individuals to confer protective immunity, typically antibody-mediated and/or cell-mediated immunity, to prevent or reduce the severity of subsequent CMV infection following a suitable immunization period.

In an aspect, the invention also provides attenuated live virus CMV vaccines wherein at least one open reading frame of a Toledo genomic region is replaced by a segment of the Towne genome which is not present in AD169. The Towne genome comprises a region not present in AD169; the region contains open reading frames designated UL147, UL152, UL 153, and UL154 and generally is spanned by nucleotides 178221 to 180029 of the Towne genome according to the AD169 (EMBL accession number X17403) numbering convention. An attenuated virus of the invention can, in one embodiment, comprise a Toledo genome wherein the Toledo genome region spanning open reading frames UL133 to UL151 are replaced with a Towne genome region spanning UL147, UL152, UL153, and UL154; this engineered CMV virus variant is an attenuated Toledo virus which comprises desirable features of Towne while reducing undesirable virulence of the Toledo genome region. The invention provides other variations of this basic method, whereby a segment of the Toledo genome region comprising at least one open reading frame is deleted or otherwise structurally disrupted in a CMV variant having a Toledo genome region or its homolog, and a segment of a Towne genome region comprising at least one open reading frame inserted in the CMV variant. In an embodiment, the engineered CMV variant comprises: (1) Toledo DNA (DNA substantially identical to a Toledo strain, preferably identical to it) from about nucleotides 1 to about 168,000 corresponding to (i.e., according to) the AD169 (EMBL accession number X17403) nucleotide numbering convention, operably linked to (2) Towne DNA (DNA substantially identical to a Towne strain, preferably identical to it) from about nucleotides 143,824 to 189,466 according to the AD169 (EMBL accession number X17403) nucleotide numbering convention, operably linked to (3) Toledo DNA (DNA substantially identical to a Toledo strain, preferably identical to it) from about nucleotides 189,466 to about 209,514 corresponding to (i.e., according to) the AD169 (EMBL accession number X17403) nucleotide numbering convention, operably linked to (4) Towne DNA (DNA substantially identical to a Towne strain, preferably identical to it) from about nucleotides 200,080 to 229,354 according to the AD169 (EMBL accession number X17403) nucleotide numbering convention. The invention also provides vaccine compositions and formulations of such attenuated CMV viruses, which can include adjuvants, delivery vehicles, liposomal formulations, and the like. The invention also provides the use of such attentuated CMV variants for prevention of CMV disease and infection; in one aspect this use includes administration of such vaccine to human subjects.

In a variation, the functional inactivation of a Toledo genomic region gene is provided by suppressing function of a gene product encoded by a Toledo region open reading frame by contacting or administering an antibody which is specifically reactive with said gene product. In an embodiment, the Toledo genomic region gene is UL148, UL141, and/or UL144, typically at least UL148, although other Toledo open reading frames can be used. The antibody binds to a gene product encoded by a Toledo region open reading frame with an affinity of at least about 1×10⁷ M⁻¹, typically at least about 1×10⁸ M⁻¹, frequently at least 1×10⁹ M⁻¹ to 1×10¹⁰ M⁻¹ or more. In some aspects, the antibody is substantially monospecific. In an embodiment, the antibody is a human antibody raised by immunizing an individual with an immunogenic dose of a gene product of a Toledo region open reading frame. In an embodiment, the human antibody is a monoclonal antibody, or collection of human monoclonal antibodies which bind to the Toledo region gene product(s) In an embodiment, the antibody is a humanized antibody comprising complementarity-determining regions substantially obtained from a non-human species immunoglobulin reactive with the Toledo region gene product, and further comprising substantially human sequence framework and constant regions. The invention also comprises pharmaceutical formulations of such antibodies and the use of such antibodies to treat or prevent CMV diseases, such as by passive immunization or the like.

In an aspect, the invention provides a composite CMV variant comprising a highly-passaged Towne genome and at least one open reading frame of a Toledo genome region, typically present in or adjacent to the U_(L)/b′ region of the composite CMV. In an aspect, the composite CMV is a highly-passaged Towne genome further comprising a Toledo UL148, UL141, and/or UL144. In an embodiment, the composite CMV is a highly-passaged Towne genome with a complete Toledo genome region; in a variation said Toledo genome region has at least one open reading frame functionally inactivated to further attenuate the virulence of the composite CMV. In a variation, a low passage Towne genome (i.e, less than 40 passages in culture) is used in place of a highly-passaged Towne genome. In an alternate variation, a virulence region from a low-passage Towne genome is emplaced in a Toledo genome so as to thereby replace at least 1 kbp of the virulence region of the Toledo genome with at least 500 bp, typically approximately the same length, of a corresponding region (e.g., substantial sequence identity) of low-passage Towne.

In an aspect, the invention provides a chimeric CMV virus, comprising a genome having a plurality of polynucleotide sequences, linked in conventional phosphodiester linkage, wherein at least two of said polynucleotide sequences are derived from different clinical isolates or strains of CMV. Said chimeric CMV virus can comprise a genome having a plurality of polynucleotide sequences, linked in conventional phosphodiester linkage, wherein a first CMV genome sequence of at least 500 bp and less than a complete CMV genome length (e.g., less than 250 kbp) is at least 98 percent sequence identical to a first CMV isolate or strain, and at least one additional CMV sequence of at least 500 bp and less than a complete CMV genome length (e.g., less than 250 kbp) is at least 98 percent sequence identical to a second CMV isolate or strain which has a genome having a polynucleotide sequence of at least 500 bp which is less than 60 percent sequence identical to any portion of the genome of said first CMV isolate or strain and/or which is absent or substantially absent in the genome of said first CMV isolate or strain. Said chimeric CMV virus comprises a genome having sufficient genetic information to replicate as a virus, typically as an infectious virus, in suitable host cells or a suitable host organism or replication system (e.g., SCID/hu thy/liv mice, human lung fibroblasts, and other systems known in the art). Generally, said chimeric CMV virus has a genome that comprises genetic information which is substantially sequence identical, generally at least 80 percent sequence identical, usually at least 95 percent sequence identical or more, to a high-passage Towne genome; said chimeric CMV virus genome typically further comprises genetic information which is substantially sequence identical, generally at least 80 percent sequence identical, usually at least 95 percent sequence identical or more, to at least 1 kbp of a virulence region of a clinical isolate of CMV or a low-passage strain of CMV other than low-passage Towne; in an embodiment, a complete virulence region (e.g., Toledo genome region) of a clinical isolate or low-passage CMV strain is present.

In an aspect, the invention provides a chimeric CMV virus, comprising a chimeric genome comprising a polynucleotide having a first CMV sequence of at least 500 bp having at least 97 percent sequence identity with a genome of a first CMV isolate or CMV strain and a second CMV sequence of at least 500 bp having at least 97 percent sequence identity with a genome of a second CMV isolate or CMV strain, and wherein said chimeric genome comprises genetic information having substantial identity (e.g., at least 80 percent sequence identity, preferably at least 95 percent sequence identity) spanning at least about the complete low-passage Towne genome. Typically, the chimeric genome comprises at least 500 bp containing at least one ORF having at least 95 to preferably 100 percent sequence identity to a virulence region (e.g., Toledo genome region) of a clinical isolate or low-passage strain of CMV other than low-passage Towne.

In an aspect, the invention provides a chimeric CMV virus, comprising a chimeric genome comprising a polynucleotide having a first CMV sequence of at least 500 bp having at least 97 percent sequence identity with a genome of a first CMV isolate or CMV strain and a second CMV sequence of at least 500 bp having at least 97 percent sequence identity with a genome of a second CMV isolate or CMV strain, and wherein said chimeric genome comprises genetic information having substantial identity (e.g., at least 80 percent sequence identity, preferably at least 95 percent sequence identity) spanning at least about the complete Toledo genome excepting at least 1 kbp of the virulence determining region of Toledo (Toledo genome region), and preferably excepting at least 5 kbp to the entire approximately 15 kbp virulence determining Toledo genome region. Typically, the chimeric genome comprises at least 500 bp containing at least one ORF having at least 95 to preferably 100 percent sequence identity to a virulence region of low-passage Towne.

In specific embodiments, the invention provides exemplary CMV chimeric viruses composed of genome portions of high-passage Towne and genome portions of Toledo; the exemplary CMV chimeric viruses are designated herein as Chimera I, Chimera II, Chimera III, Chimera IV, and Towne/Tol 11. In an aspect, the invention encompasses these specific embodiments and variants of each exemplified Chimera wherein the boundaries (splice junctions/recombination joints) between the various Towne and Toledo genome portions vary from the specific exemplified Chimeras by less than 20 kbp, typically less than 10 kbp, usually by less than 5 kbp, and in many embodiments by less than 1 kbp from the specific examples provided herein.

In a variation, the invention provides a diagnostic method for identifying a virulent CMV strain in a sample by detecting the presence of unique Toledo genome region polynucleotide sequences and/or by detecting the presence of a polypeptide encoded by an open reading frame of the Toledo genomic region. Detection of polynucleotide sequences can be by any suitable method, including but not limited to PCR amplification using suitable primers, LCR, hybridization of a labeled polynucelotide probe, and the like. Detection of polypeptide species is typically done by immunoassay using a specific antibody to the Toledo region gene product(s).

The invention also provides a method of treating or preventing CMV infection, the method comprising administering to an individual an efficacious dose of a polypeptide which is substantially identical to the deduced amino acid sequence of UL148. In a variation, the polypeptide is a truncated variant, mutein, or analog of the deduced amino acid sequence of UL148, wherein the polypeptide is soluble.

A further understanding of the nature and advantages of the invention will become apparent by reference to the remaining portions of the specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1R. Nucleotide sequence of Toledo genome region isolated from Toledo strain of HCMV. (SEQ ID NO: 1)

FIGS. 2A-2H. Deduced amino acid sequences of open reading frames UL130, and UL132 through UL151 (SEQ ID NOs:2-22, respectively). Conventional single letter abbreviations are used.

FIG. 3. Schematic representation of open reading frames and their location in Toledo genome region. Top line schematically portrays entire Toledo genome with U_(L)/b′ region identified. Bottom line shows enlarged view of U_(L)/b′ region. Arrows indicate polarity and length of open reading frame. Solid circles indicate potential glycosylation sites.

FIG. 4. Schematic comparison of the novel genome regions of Toledo and highly-passaged Towne as compared to AD169.

FIG. 5. CMV Towne and Toledo cosmids used to regenerate specific chimeric CMV viruses. The location of the cosmid insert are indicated beneath the appropriate viral genome. The numbers at the end of the insert denote the endpoints determined by DNA sequence analysis; the numbers correspond to AD169 genomic sequence in GenBank (EMBL accession number X17403). “XXX” denotes an end which was refractory to DNA sequence analysis. These ends were mapped by restriction enzyme and Southern blot analyses. The vertical dashed line represents the location of the internal “a” sequence of the virus. The lower line depicts the structure of the Tol/Twn 39/50 genome. The thick gray line denotes sequences derived from Toledo and the thin black line depicts sequences contributed from highly-passaged Towne strain. Regions of overlap could be derived from either virus and are represented by a region of a thick gray and a thin black line together. The Tol/Twn 39/50 genome does not contain the Toledo genomic region.

FIG. 6. Analysis of the gpt/LacZ recombinant viruses in the SCID-hu (thy/liv) model. Two independent isolates of Tol pGD6 and Tol pGD7 were tested in the model. 3 mice were used per group and the mean of the data is displayed. Error bars representing 2 standard errors from the mean are also displayed.

FIG. 7. Southern blot showing that a variety of clinical isolates of CMV contain sequences homologous to the Toledo U_(L)/b′ region. The Towne lane contains genomic DNA from Aviron's highly-passaged Towne strain (Towne AV).

FIG. 8. Southern blot showing that previous variants of the Towne strain hybridize to the Toledo U_(L)/b′ region. Twn•Merck indicates Towne strain from the Merck clinical trial. Twn•MA, Twn•MA#5 and Twn•MA#8 are variants of Towne obtained from Microbiological Associates. Twn•Aviron is highly-passaged Towne obtained at Aviron.

FIG. 9. Schematic depiction of generation of chimeric CMV virus genomes by cotransfection of cosmids containing portions of Towne and Toledo genomes.

FIG. 10. Schematic depiction of the specific exemplary embodiments denoted Chimera I, Chimera II, Chimera III, Chimera IV, and Towne/Tol 11. Toledo genome is depicted as “Toledo”; highly-passaged Towne genome is depicted as “Towne•AV”; selected reading frames of importance, proposed function/homologues of selected ORFs, and scale (in kbp) is shown on the top line.

FIG. 11. Replication of Toledo, highly passaged Towne, and Chimeras I, III, and IV (in order, respectively) in SCID-hu mice having a thymus/liver implant.

FIG. 12. Schematic comparison of low-passage (long) Towne genome and high-passage (short) Towne genome.

DEFINITIONS

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described. For purposes of the present invention, the following terms are defined below.

As used herein, the twenty conventional amino acids and their abbreviations follow conventional usage (Immunology—A Synthesis, 2nd Edition, E. S. Golub and D. R. Gren, Eds., Sinauer Associates, Sunderland, Mass. (1991)). Stereoisomers (e.g., D-amino acids) of the twenty conventional amino acids, unnatural amino acids such as α,α-disubstituted amino acids, N-alkyl amino acids, lactic acid, and other unconventional amino acids may also be suitable components for polypeptides of the present invention. Examples of unconventional amino acids include: 4-hydroxyproline, γ-carboxyglutamate, ε-N,N,N-trimethyllysine, ε-N-acetyllysine, O-phosphoserine, N-acetylserine, N-formylmethionine, 3-methylhistidine, 5-hydroxylysine, ω-N-methylarginine, and other similar amino acids and imino acids (e.g., 4-hydroxyproline). In the polypeptide notation used herein, the lefthand direction is the amino terminal direction and the righthand direction is the carboxy-terminal direction, in accordance with standard usage and convention. Similarly, unless specified otherwise, the lefthand end of single-stranded polynucleotide sequences is the 5′ end; the lefthand direction of double-stranded polynucleotide sequences is referred to as the 5′ direction. The direction of 5′ to 3′ addition of nascent RNA transcripts is referred to as the transcription direction; sequence regions on the DNA strand having the same sequence as the RNA and which are 5′ to the 5′ end of the RNA transcript are referred to as “upstream sequences”; sequence regions on the DNA strand having the same sequence as the RNA and which are 3′ to the 3′ end of the coding RNA transcript are referred to as “downstream sequences”.

The term “naturally-occurring” as used herein as applied to an object refers to the fact that an object can be found in nature. For example, a polypeptide or polynucleotide sequence that is present in an organism (including viruses) that can be isolated from a source in nature and which has not been intentionally modified by man in the laboratory is naturally-occurring. Generally, the term naturally-occurring refers to an object as present in a non-pathological (undiseased) individual, such as would be typical for the species.

The term “corresponds to” is used herein to mean that a polynucleotide sequence is homologous (i.e., is identical, not strictly evolutionarily related) to all or a portion of a reference polynucleotide sequence, or that a polypeptide sequence is identical to a reference polypeptide sequence. In contradistinction, the term “complementary to” is used herein to mean that the complementary sequence is homologous to all or a portion of a reference polynucleotide sequence. For illustration, the nucleotide sequence “TATAC” corresponds to a reference sequence “TATAC” and is complementary to a reference sequence “GTATA”.

The following terms are used to describe the sequence relationships between two or more polynucleotides: “reference sequence”, “comparison window”, “sequence identity”, “percentage of sequence identity”, and “substantial identity”. A “reference sequence” is a defined sequence used as a basis for a sequence comparison; a reference sequence may be a subset of a larger sequence, for example, as a segment of a full-length cDNA or gene sequence given in a sequence listing, such as a polynucleotide sequence of FIG. 1A-1R (SEQ ID NO. 1), or may comprise a complete cDNA or gene sequence. A full-length cDNA or gene sequence is defined as a polynucleotide containing the sequence(s) necessary to encode a complete protein product, including a translation initiation codon and a translation termination codon, unless linked to another encoding sequence in a format for production as a fusion protein. Generally, a reference sequence is at least 20 nucleotides in length, frequently at least 25 nucleotides in length, and often at least 50 nucleotides in length. Since two polynucleotides may each (1) comprise a sequence (i.e., a portion of the complete polynucleotide sequence) that is similar between the two polynucleotides, and (2) may further comprise a sequence that is divergent between the two polynucleotides, sequence comparisons between two (or more) polynucleotides are typically performed by comparing sequences of the two polynucleotides over a “comparison window” to identify and compare local regions of sequence similarity.

A “comparison window”, as used herein, refers to a conceptual segment of at least 20 contiguous nucleotide positions wherein a polynucleotide sequence may be compared to a reference sequence of at least 20 contiguous nucleotides and wherein the portion of the polynucleotide sequence in the comparison window may comprise additions or deletions (i.e., gaps) of 20 percent or less as compared to the reference sequence (which does not comprise additions or deletions) for optimal alignment of the two sequences. Optimal alignment of sequences for aligning a comparison window may be conducted by the local homology algorithm of Smith and Waterman (1981) Adv. Appl. Math. 2: 482, by the homology alignment algorithm of Needleman and Wunsch (1970) J. Mol. Biol. 48: 443, by the search for similarity method of Pearson and Lipman (1988) Proc. Natl. Acad. Sci. (U.S.A.) 85: 2444, by computerized implementations of these algorithms (GAP, BESTFIT, FASTA, and TFASTA in the Wisconsin Genetics Software Package Release 7.0, Genetics Computer Group, 575 Science Dr., Madison, Wis.), or by inspection, and the best alignment (i.e., resulting in the highest percentage of homology over the comparison window) generated by the various methods is selected.

The term “sequence identity” means that two polynucleotide sequences are identical (i.e., on a nucleotide-by-nucleotide basis) over the window of comparison. The term “percentage of sequence identity” is calculated by comparing two optimally aligned sequences over the window of comparison, determining the number of positions at which the identical nucleic acid base (e.g., A, T, C, G, U, or I) occurs in both sequences to yield the number of matched positions, dividing the number of matched-positions by the total number of positions in the window of comparison (i.e., the window size), and multiplying the result by 100 to yield the percentage of sequence identity. The terms “substantial identity” as used herein denotes a characteristic of a polynucleotide sequence, wherein the polynucleotide comprises a sequence that has at least 80 percent sequence identity, preferably at least 85 percent identity and often 90 to 95 percent sequence identity, more usually at least 99 percent sequence identity as compared to a reference sequence over a comparison window of at least 20 nucleotide positions, frequently over a window of at least 25-50 nucleotides, wherein the percentage of sequence identity is calculated by comparing the reference sequence to the polynucleotide sequence which may include deletions or additions which total 20 percent or less of the reference sequence over the window of comparison. The reference sequence may be a subset of a larger sequence, for example, as a segment of an open-reading frame shown in FIG. 1A-1R.

As applied to polypeptides, the term “substantial identity” means that two peptide sequences, when optimally aligned, such as by the programs GAP or BESTFIT using default gap weights, share at least 80 percent sequence identity, preferably at least 90 percent sequence identity, more preferably at least 95 percent sequence identity or more (e.g., 99 percent sequence identity). Preferably, residue positions which are not identical differ by conservative amino acid substitutions.

Conservative amino acid substitutions refer to the interchangeability of residues having similar side chains. For example, a group of amino acids having aliphatic side chains is glycine, alanine, valine, leucine, and isoleucine; a group of amino acids having aliphatic-hydroxyl side chains is serine and threonine; a group of amino acids having amide-containing side chains is asparagine and glutamine; a group of amino acids having aromatic side chains is phenylalanine, tyrosine, and tryptophan; a group of amino acids having basic side chains is lysine, arginine, and histidine; and a group of amino acids having sulfur-containing side chains is cysteine and methionine. Preferred conservative amino acids substitution groups are: valine-leucine-isoleucine, phenylalanine-tyrosine, lysine-arginine, alanine-valine, and asparagine-glutamine.

The term “analog”, “mutein” or “mutant” as used herein refers to polypeptides which are comprised of a segment of at least 10 amino acids that has substantial identity to a portion of the naturally occurring protein

The term “cognate” as used herein refers to a gene sequence that is evolutionarily and functionally related between species. For example but not limitation, in the human genome, the human CD4 gene is the cognate gene to the mouse CD4 gene, since the sequences and structures of these two genes indicate that they are highly homologous and both genes encode a protein which functions in signaling T cell activation through MHC class II-restricted antigen recognition.

The term ‘agent’ is used herein to denote a chemical compound, a mixture of chemical compounds, an array of spatially localized compounds (e.g., a VLSIPS peptide array, polynucleotide array, and/or combinatorial small molecule array), a biological macromolecule, a bacteriophage peptide display library, a bacteriophage antibody (e.g., scFv) display library, a polysome peptide display library, or an extract made from biological materials such as bacteria, plants, fungi, or animal (particularly mammalian) cells or tissues. Agents are evaluated for potential activity as antineoplastics, anti-inflammatories, or apoptosis modulators by inclusion in screening assays described hereinbelow. Agents are evaluated for potential activity as specific protein interaction inhibitors (i.e., an agent which selectively inhibits a binding interaction between two predetermined polypeptides but which does not substantially interfere with cell viability) by inclusion in screening assays.

As used herein, the terms “label” or “labeled” refers to incorporation of a detectable marker, e.g., by incorporation of a radiolabeled amino acid or attachment to a polypeptide of biotinyl moieties that can be detected by marked avidin (e.g., streptavidin containing a fluorescent marker or enzymatic activity that can be detected by optical or calorimetric methods). Various methods of labeling polypeptides and glycoproteins are known in the art and may be used. Examples of labels for polypeptides include, but are not limited to, the following: radioisotopes (e.g., ³H, ¹⁴C, ³⁵S, ¹²⁵I, ¹³¹I), fluorescent labels (e.g., FITC, rhodamine, lanthanide phosphors), enzymatic labels (e.g., horseradish peroxidase, β-galactosidase, luciferase, alkaline phosphatase), biotinyl groups, predetermined polypeptide epitopes recognized by a secondary reporter (e.g., leucine zipper pair sequences, binding sites for secondary antibodies, transcriptional activator polypeptide, metal binding domains, epitope tags). In some embodiments, labels are attached by spacer arms of various lengths to reduce potential steric hindrance.

As used herein, “substantially pure” means an object species is the predominant species present (i.e., on a molar basis it is more abundant than any other individual macromolecular species in the composition), and preferably a substantially purified fraction is a composition wherein the object species comprises at least about 50 percent (on a molar basis) of all macromolecular species present. Generally, a substantially pure composition will comprise more than about 80 to 90 percent of all macromolecular species present in the composition. Most preferably, the object species is purified to essential homogeneity (contaminant species cannot be detected in the composition by conventional detection methods) wherein the composition consists essentially of a single macromolecular species. Solvent species, small molecules (<500 Daltons), and elemental ion species are not considered macromolecular species.

The term “primer” as used herein refers to an oligonucleotide whether occurring naturally as in a purified restriction digest or produced synthetically, which is capable of acting as a point of initiation of synthesis when placed under conditions in which synthesis of a primer extension product which is complementary to a nucleic acid strand is induced, i.e., in the presence of nucleotides and an agent for polymerization such as DNA polymerase and at a suitable temperature and pH. The primer is preferably single-stranded for maximum efficiency in amplification, but may alternatively be double stranded. If double stranded, the primer is first treated to separate its strands before being used to prepare extension products. Preferably, the primer is an oligodeoxyribonucleotide. The primer must be sufficiently long to prime the synthesis of extension products in the presence of the agent for polymerization. The exact lengths of the primers will depend on many factors, including temperature and source of primers. For example, depending on the complexity of the target sequence, the oligonucleotide primer typically contains 15-25 or more nucleotides, although it may contain fewer nucleotides. Short primer molecules generally require cooler temperatures to form sufficiently stable hybrid complexes with template. In some embodiments, the primers can be large polynucleotides, such as from about 200 nucleotides to several kilobases or more. The primers herein are selected to be substantially complementary to the different strands of each specific sequence to be amplified. The primers must be sufficiently complementary to hybridize with their respective strands. Therefore, the primer sequence-need not reflect the exact sequence of the template. For example, a non-complementary nucleotide fragment may be attached to the 5′ end of the primer, with the remainder of the primer sequence being complementary to the strand. Alternatively, noncomplementary bases or longer sequences can be interspersed into the primer, provided that the primer sequence has sufficient complementarity with the sequence of the strand to be amplified to hybridize therewith and thereby form a template for synthesis of the extension product of the other primer.

The term “recombinant” used herein refers to macromolecules produced by recombinant DNA techniques wherein the gene coding for a polypeptide is cloned by known recombinant DNA technology. For example, an amplified or assembled product polynucleotide may be inserted into a suitable DNA vector, such as a bacterial plasmid, and the plasmid used to transform a suitable host. The gene is then expressed in the host to produce the recombinant protein. The transformed host may be prokaryotic or eukaryotic, including mammalian, yeast, Aspergillus and insect cells. One preferred embodiment employs bacterial cells as the host. Alternatively, the product polynucleotide may serve a non-coding function (e.g., promoter, origin of replication, ribosome-binding site, etc.).

DETAILED DESCRIPTION

Commonly-assigned U.S. patent application U.S. Ser. No. 08/414,926 filed 31 Mar. 1995 is incorporated herein by reference.

The nomenclature used hereafter and the laboratory procedures in cell culture, molecular genetics, and nucleic acid chemistry and hybridization described below may involve well known and commonly employed procedures in the art. Standard techniques are used for recombinant nucleic acid methods, polynucleotide synthesis, and microbial culture and transformation (e.g., electroporation, lipofection). The techniques and procedures are generally performed according to conventional methods in the art and various general references (see, generally, Sambrook et al. Molecular Cloning: A Laboratory Manual, 2d ed. (1989) Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.

Oligonucleotides can be synthesized on an Applied Bio Systems oligonucleotide synthesizer according to specifications provided by the manufacturer.

Methods for PCR amplification are described in the art (PCR Technology: Principles and Applications for DNA Amplification ed. HA Erlich, Stockton Press, New York, N.Y. (1989); PCR Protocols: A Guide to Methods and Applications, eds. Innis, Gelfland, Snisky, and White, Academic Press, San Diego, Calif. (1990); Mattila et al. (1991) Nucleic Acids Res. 19: 4967; Eckert, K. A. and Kunkel, T. A. (1991) PCR Methods and Applications 1: 17; and U.S. Pat. Nos. 4,683,202 and 4,965,188, each of which are incorporated herein by reference) and exemplified hereinbelow.

It is evident that optimal PCR and hybridization conditions will vary depending upon the sequence composition and length(s) of the targeting polynucleotide(s) and target(s), and the experimental method selected by the practitioner. Various guidelines may be used to select appropriate primer sequences and hybridization conditions (see, Maniatis et al., Molecular Cloning: A Laboratory Manual (1989), 2nd Ed., Cold Spring Harbor, N.Y.; Berger and Kimmel, Methods in Enzymology, Volume 152. Guide to Molecular Cloning Techniques (1987), Academic Press, Inc., San Diego, Calif.; PCR Protocols: A Guide to Methods and Applications, eds. Innis, Gelfland, Snisky, and White, Academic Press, San Diego, Calif. (1990); Benton W D and Davis R W (1977) Science 196: 180; Goodspeed et al. (1989) Gene 76: 1; Dunn et al. (1989) J. Biol. Chem. 264: 13057 which are incorporated herein by reference.

A basis of the invention is the unexpected discovery that there are significant genomic differences between clinical isolates of CMV and highly-passaged CMV strains, including differences between low-passage Towne and high-passage Towne, as well as differences as compared to Toledo strain; the identification of these genomic differences, including definition of novel genomic region(s); and the phenotypic significance and biological function of said genomic differences and specific ORFs within said novel genomic regions. Based, in part, on these unexpected discoveries, it is possible to construct and use chimeric CMV viruses which have predetermined genome compositions comprising at least a portion of a genome of a first CMV isolate or strain and at least a portion of a genome of a second (or subsequent) CMV isolate or strain, so as to form a complete, replicable recombinant chimeric CMV genome, with and the resultant chimeric CMV genome being capable of replication in a suitable host replication system and being useful for a variety of uses, such as human or veterinary vaccines, commercial reagents for laboratory use (e.g., as restriction enzymes are sold), use in screening systems to identify novel candidate drugs to inhibit replication or pathogenesis (e.g., virulence, tropism, host range, etc.) of pathogenic, clinically relevant CMV virus types, and other uses such as diagnostic reagents, gene expression vectors, anti-tumor agents, heterologous gene expression systems, and the like.

Overview

An approach of the invention starts with identification of DNA sequences which confer virulence on human cytomegalovirus (HCMV). These sequences can be manipulated to produce a new, more efficacious HCMV vaccine strain with predicted characteristics. Introduction of the virulence genes into an overattenuated strain can improve its immunogenicity and deletion of the virulence genes from a virulent strain can render it safe in humans by decreasing its virulence. Specifically, deletion of genetic information from a clinical isolate called Toledo is used to attenuate an HCMV virus, and in one embodiment, a segment from a laboratory strain called Towne, especially a highly-passaged Towne variant, is transferred to the deleted region of Toledo to act as a “spacer”. Deleting genetic information has utility in improving a clinical isolate such as Toledo as an immunizing composition. Removing these sequences from Toledo, which has been shown to cause disease in people, can result in an attenuated virus which may be a safe vaccine candidate.

The Towne strain of HCMV has been used as a vaccine in humans. In some clinical settings, Towne has been used to prevent the disease consequences associated with infection by HCMV (reviewed by Marshall and Plotkin In: The Human Herpesviruses B. Roizman, R. J. Whitley, & C. Lopez Eds. Raven Press, New York). The Towne strain is believed to be overattenuated as a vaccine candidate and consequently, is poorly immunogenic. This loss of immunogenicity may have been the result of an extensive passage history in tissue culture. Genetic information in the virulence region may have been lost during passage, particularly after about Passage 40. Variation in DNA content among isolated strains does exist based on crude hybridization experiments. Other investigators have reported minor regions of sequence heterogeneity between two so-called laboratory strains of HCMV, the Towne and AD169 strains. Heterogeneities can exist within HCMV strains depending upon the extent of passages in their culture history.

The public health impact of HCMV infections have not been well controlled by current treatment strategies or available antiviral chemotherapies. Preventive vaccine strategies are likely to prove efficacious because of the observations that seropositive renal allograft recipients are protected from severe HCMV disease and maternal immunity protects the fetus from disease after intrauterine infection. HCMV (Towne) was developed as a vaccine strain by serial passage 125 times in WI38 human diploid fibroblasts (Towne 125). It has been administered to humans without significant adverse reactions. However, in one study, vaccinees were directly challenged by wild-type virus and found to resist only low challenge doses of 10 plaque-forming units or less. The consensus view is that the Towne strain may be overly attenuated. One positive feature of the Towne strain is that it has never been shown to reactivate.

One important obstacle to the development of a vaccine for HCMV is the lack of an animal model system that can be used to test the safety and efficacy of vaccine candidates. Therefore, cell culture systems or surrogate animal models such as the SCID-hu (thy/liv) mouse have to be developed to test vaccine strains. Replicative differences in HCMV strains have been described in a variety of cell types and in the SCID-hu mouse model. These differences correlate to the virulence and passage history of the virus. Thus, low passage, virulent clinical isolates, such as Toledo, can replicate better in the human implant of SCID-hu (thy/liv) mice and in cultures of human endothelial cells than cell culture adapted, highly-passaged avirulent laboratory strains such as Towne or AD169 (Brown et al. 1995; Waldman et al., 1991). This observation can be exploited to measure the “virulence” of a strain by assessing its growth characteristics in the SCID-hu mouse, in vivo in humans, or by other means. Recombinant vaccine candidates such as the ones described here which have deleted or incorporated DNA sequences are believed to replicate less well than the virulent parent in a suitable virulence assay. This observation would be indicative of an attenuated vaccine candidate. Deletion of the Toledo U_(L)/b′ region from the low passage, virulent HCMV Toledo genome results in a virus with reduced replicative ability in the SCID-hu mouse. This recombinant virus should have a concomitantly reduced virulence which allows administration of the virus without causing the undesired clinical manifestations exhibited by the Toledo virus in humans.

The invention identifies, maps, and sequences differences between the virulent Toledo strain and the avirulent highly passaged Towne strain, for the purpose of transferring novel genetic information to Towne to restore its immunogenicity or, alternatively, to remove information from Toledo to render it safe as a vaccine candidate. One major region of difference mapped to the internal portion of the L component. This large 13 kbp region present in Toledo but not highly passaged Towne is located at the border between the unique long (UL) and the inverted repeats bordering the UL region termed IRL or b′. We have deduced the coding information resident in the Toledo sequences and have extensively compared the information resident in AD169, highly passaged Towne and Toledo. We have made recombinant viruses which have either inserted the U_(L)/b′ region from the virulent Toledo strain, into the corresponding region of Towne, and have also deleted this region from Toledo and replaced it with a selectable marker and reporter gene or with the corresponding U_(L)/b′ region from Towne. Deletion of the virulence genes from Toledo decreased the ability of the recombinant to replicate within the SCID-hu (thy/liv) mouse, a model for CMV virulence. The new recombinant viruses exhibit growth properties in the SCID-hu mouse that indicate that vaccine candidates with attenuated virulence can be generated by deleting the UL/b′ region from the Toledo virus. We have also demonstrated that we can add the Toledo region to the Towne virus which will presumably result in increased immunogenicity for the highly w passaged Towne virus while retaining its safe profile for humans.

FIGS. 1A-1R show the nucleotide sequence of Toledo genome region isolated from Toledo strain of HCMV (SEQ ID NO. 1). FIGS. 2A-2H show the deduced amino acid sequences of open reading frames UL130, and UL132 through UL151 (SEQ ID NOS 2-22, respectively).

A basis of the present invention is the surprising and unexpected finding that: (1) clinical isolates of pathogenic CMV variants contain a genomic region which typically is not present in CMV strains which have undergone extensive laboratory passaging of the virus in cell culture, and (2) functional disruption (e.g., deletion or insertional inactivation and the like) of genes in this genomic region produces a substantial attenuation of CMV virulence and pathogenicity in vivo. The genomic region is conveniently termed the “Toledo genomic region” herein, although equivalent (e.g., homologous) regions or subsequences thereof are present in other clinical isolates of CMV besides the Toledo strain of CMV; the term “Toledo genomic region” encompasses these homologous regions in other clinical CMV isolates and non-isolated pathogenic CMV variants which have a genomic region of at least 500 bp having at least 80 percent sequence identity to the Toledo genomic region of the Toledo strain having the sequences disclosed herein and in WO96/30387, incorporated herein by reference. The Toledo genomic region which is present in pathogenic CMV isolates and which is typically substantially absent in laboratory passaged CMV strains (e.g., AD169, Towne) has been sequenced and several open-reading frames have been identified. Functional disruption of these open reading frames, either singly or in combination, has been unexpectedly found to substantially reduce virulence of the resultant CMV mutant(s) in vivo. Thus, in part, the invention provides methods and compositions for suppressing or inactivating expression of genes of the Toledo genomic region and its homolog regions in other CMV variants, and thereby reducing virulence and pathogenicity of clinically important CMV variants. The LB invention is, in part, further based on the heretofore unrecognized finding that pathogenic clinical isolates of CMV have a distinct genome as compared to the commonly used laboratory-passaged strains of human CMV (e.g., AD169, Towne), and that the genomic region which is present in the clinical isolates and which is substantially absent in laboratory-passaged strains confers enhanced virulence in vivo. Most common approaches to development of CMV therapies and vaccines have heretofore relied on laboratory-passaged strains which lack the Toledo genomic region and the genes encoded therein which have been unexpectedly found to confer enhanced in vivo virulence and are believed to contribute to clinical pathology and CMV-related disease.

The invention provides a method for attenuating virulence of CMV comprising functionally inactivating at least one open reading frame in a genomic region of a CMV genome having substantial sequence identity to at least 300 bp, typically at least 500 bp, of an approximately 15 kb sequence present in the genome of the Toledo strain of CMV and absent from the genome of the AD169 strain of CMV. In an aspect, the method functionally inactivates at least one open reading frame present in a genomic region of a CMV genome having substantial identity to at least 300 bp of a 13 kb sequence present in the genome of the Toledo strain of CMV and absent from the genome of the highly-passaged Towne strain of CMV. In an embodiment, the method functionally inactivates at least one open reading frame present in a genomic region of a CMV genome having substantial identity to at least 500 bp of the sequence shown in FIGS. 1A through 1R (SEQ ID NO: 1). In an embodiment, the method functionally inactivates at least the open reading frame corresponding to UL148 as identified herein. In a variation, the method functionally inactivates open reading frames in the region spanning UL138 to UL148. In an embodiment, the method functionally inactivates UL138, UL139, UL140, UL141, UL142, UL143, UL144, UL145, UL146, UL147, and/or UL148. In a variation, UL148 is inactivated singly or in combination with other open reading frames of the Toledo genomic region. In a specific embodiment, UL148 is inactivated in combination with UL141 and/or UL144. Inactivation is typically accomplished by genetic engineering and involves predetermined mutations (which may include additions, transpositions, or deletions), generally of the specific type which are not known to occur naturally in CMV strains even after extensive passaging.

In an aspect, the method of attenuating virulence comprises functional inactivation of open reading frames by structural mutation (e.g., deletion, insertion, missense or nonsense mutation, and the like) of at least one open reading frame, or a mutation of a transcriptional control sequence that controls transcription of the open reading frame, or mutation of a splicing signal sequence or the like necessary for efficient expression of the encoded gene product of the open reading frame. In an embodiment, a selectable marker gene is introduced into an open reading frame, often in the portion of the open reading frame believed to encode the amino-terminal two-thirds of the gene product, to structurally disrupt the open reading frame and result in the inactivation of the open reading frame's capacity to encode its functional gene product. In a variation, open reading frame UL148 is structurally disrupted by predetermined mutation, often produced by site-directed mutagenesis or in vitro recombination; in one embodiment the structural disruption results from insertion of a selectable and/or screenable marker gene (e.g., gpt/lacZ). In an embodiment, a selectable marker gene is used to replace all or part of at least one open reading frame, such as by replacement of a deleted region of the Toledo genomic region with a selectable marker gene. In a variation, a region spanning open reading frame UL138 to UL148 is structurally disrupted by predetermined mutation; in one embodiment the structural disruption results from deletion of the UL138-UL148 region and replacement with a selectable and/or screenable marker gene (e.g., gpt/lacZ).

In an aspect, the functional inactivation of a Toledo genomic region gene is provided by transcriptional and/or translational suppression with an antisense polynucleotide having a sequence of at least 15 nucleotides, typically at least 25 nucleotides, that are substantially complementary to a Toledo genomic region, most usually the antisense polynucleotide is substantially complementary to an open reading frame sequence of a Toledo genomic region open reading frame. In an embodiment, the antisense polynucleotide is substantially complementary to at least 25 nucleotides of UL148. In an embodiment, the antisense polynucleotide is complementary to UL148 and further comprises additional 5′ and/or 3′ nucleotide(s) which are not substantially complementary to UL148. In variations, the antisense polynucleotides comprise non-natural chemical modifications, and can include, for instance, methylphosphonates, phosphorothioates, phosphoramidites, phosphorodithioates, phosphorotriesters, and boranophosphates. In a variation the antisense molecules can comprise non-phosphodiester polynucleotide analogs wherein the phosphodiester backbone is replaced by a structural mimic linkage include: alkanes, ethers, thioethers, amines, ketones, formacetals, thioformacetals, amides, carbamates, ureas, hydroxylamines, sulfamates, sulfamides, sulfones, and glycinylamides. In a variation, the invention provides peptide nucleic acids (PNAs) having a nucleobase sequence which is substantially-complementary to a Toledo genomic region sequence, such as an open reading frame (e.g., UL148, UL141, UL142, etc.).

The invention also provides attenuated live virus CMV vaccines wherein at least one open reading frame of a Toledo genomic region is structurally disrupted by predetermined mutation. Typically, the UL148 open reading frame is structurally disrupted, either singly or in combination with other Toledo region open reading frames (e.g., UL141, UL144, and g the like). Often the disruption of the open reading frame is an insertion, deletion, or replacement mutation which confers the property of reduced virulence as determined by a suitable in vivo virulence assay (e.g., see Experimental Examples). Toledo N genomic region mutants which exhibit at least one log reduction, N preferably two logs or more reduction, in virulence as determined by in vivo virulence assay, or other equivalent virulence measure, are attenuated CMV vaccines. Such attenuated CMV vaccines are used to immunize individuals to confer protective immunity, typically antibody-mediated and/or cell-mediated immunity, to prevent or reduce the severity of subsequent CMV infection following a suitable immunization period.

In an aspect, the invention also provides attenuated live virus CMV vaccines wherein at least one open reading frame of a Toledo genomic region is replaced by a segment of the Towne genome which is not present in AD169. The highly-passaged Towne genome comprises a region not present in AD169; the region contains open reading frames designated UL147, UL152, UL 153, and UL154 and generally is spanned by nucleotides 178221 to 180029 of the Towne genome according to the AD169 (EMBL accession number X17403) numbering convention. An attenuated virus of the invention can, in one embodiment, comprise a Toledo genome wherein the Toledo genome region spanning open reading frames UL133 to UL151 are replaced with a Towne genome region spanning UL147, UL152, UL153, and UL154; this engineered CMV virus variant is an attenuated Toledo virus which comprises desirable features of Towne while reducing undesirable virulence of the Toledo genome region. The invention provides other variations of this basic method, whereby a segment of the Toledo genome region comprising at least one open reading frame is deleted or otherwise structurally disrupted in a CMV variant having a Toledo genome region or its homolog, and a segment of a Towne genome region comprising at least one open reading frame in inserted in the CMV variant. In an embodiment, the engineered CMV variant comprises: (1) Toledo DNA (DNA substantially identical to a Toledo strain, preferably identical to it) from about nucleotides 1 to about 168,000 corresponding to (i.e., according to) the AD169 (EMBL accession number X17403) nucleotide numbering convention, operably linked to (2) Towne DNA (DNA substantially identical to a Towne strain, preferably identical to it) from about nucleotides 143,824 to 189,466 according to the AD169 (EMBL accession number X17403) nucleotide numbering convention, operably-linked to (3) Toledo DNA (DNA substantially identical to a Toledo strain, preferably identical to it) from about nucleotides 189,466 to about 209,514 corresponding to (i.e., according to) the AD169 (EMBL accession number X17403) nucleotide numbering convention, operably linked to (4) Towne DNA (DNA substantially identical to a Towne strain, preferably identical to it) from about nucleotides 200,080 to 229,354 according to the AD169 (EMBL accession number X17403) nucleotide numbering convention. The invention also provides vaccine compositions and formulations of such attenuated CMV viruses, which can include adjuvants, delivery vehicles, liposomal formulations, and the like. The invention also provides the use of such attentuated CMV variants for prevention of CMV disease and infection; in one aspect this use includes administration of such vaccine to human subjects.

In a variation, the functional inactivation of a Toledo genomic region gene is provided by suppressing function of a gene product encoded by a Toledo region open reading frame by contacting or administering an antibody which is specifically reactive with said gene product. In an embodiment, the Toledo genomic region gene is UL148, UL141, and/or UL144, typically at least UL148, although other Toledo open reading frames can be used. The antibody binds to a gene product encoded by a Toledo region open reading frame with an affinity of at least about 1×10⁷ M⁻¹, typically at least about 1×10⁸ M⁻¹, frequently at least 1×10⁹ M⁻¹ to 1×10¹⁰ M⁻¹ or more. In some aspects, the antibody is substantially monospecific. In an embodiment, the antibody is a human antibody raised by immunizing an individual with an immunogenic dose of a gene product of a Toledo region open reading frame. In an embodiment, the human antibody is a monoclonal antibody, or collection of human monoclonal antibodies which bind to the Toledo region gene product(s). In an embodiment, the antibody is a humanized antibody comprising complementarity-determining regions substantially obtained from a non-human species immunoglobulin reactive with the Toledo region gene product, and further comprising substantially human sequence framework and constant regions. The invention also comprises pharmaceutical formulations of such antibodies and the use of such antibodies to treat or prevent CMV diseases, such as by passive immunization or the like.

In an aspect, the invention provides a composite CMV variant comprising a Towne genome and at least one open reading frame of a Toledo genome region; typically present in or adjacent to the U_(L)/b′ region of the composite CMV. In an aspect, the composite CMV is a Towne genome further comprising a Toledo UL148, UL141, and/or UL144. In an embodiment, the composite CMV is a highly-passaged Towne genome with a complete Toledo genome region; in a variation said Toledo genome region has at least one open reading frame functionally inactivated to further attenuate the virulence of the composite CMV.

In a variation, the invention provides a diagnostic method for identifying a virulent CMV strain in a sample by detecting the presence of unique Toledo genome region polynucleotide sequences and/or by detecting the presence of a polypeptide encoded by an open reading frame of the Toledo genomic region. Detection of polynucleotide sequences can be by any suitable method, including but not limited to PCR amplification using suitable primers, LCR, hybridization of a labeled polynucelotide probe, and the like. Detection of polypeptide species is typically done by immunoassay using a specific antibody to the Toledo region gene product(s).

The invention also provides a method of treating or preventing CMV infection, the method comprising administering to an individual an efficacious dose of a polypeptide which is substantially identical to the deduced amino acid sequence of UL148. In a variation, the polypeptide is a truncated variant, mutein, or analog of the deduced amino acid sequence of UL148, wherein the polypeptide is soluble.

Experimental Examples Overview

The growth advantage of Toledo in the SCID-hu mouse model resides in the genetic information encoded by the additional sequences (Toledo genomic region) we have identified. One gene in particular, UL148, has been mutagenized in Toledo by insertion of a selectable marker (gptILacZ) and the Toledo-based recombinant has been shown to replicate less well than Toledo in the SCID-hu assay. The genetic information of the corresponding region of the avirulent Towne virus has been deduced by nucleotide sequence analysis and demonstrated to lack an open reading frame in Towne. UL148 can be considered to be representative of a “virulence determinant” for Toledo. The new Toledo sequence identified at the inverted repeats has been analyzed to reveal novel genes in Toledo. Deletion of genes encompassing UL138 to UL148 in recombinant viruses have been tested for growth properties in the SCID-hu (thy/liv) mouse. These recombinants have been shown to replicate to levels similar to the Towne virus and represent attenuated vaccine candidates, since Towne has been shown to be safe and avirulent in humans. Such recombinants should show increased immunogenicity owing to their greater similarity to low passage virulent strains over that shown by highly-passaged Towne in humans. In addition, these strains should not exhibit the fully virulent phenotype shown by unmodified Toledo in humans due to the alterations we have introduced into their genomes.

This invention describes new recombinant HCMV viruses not previously described which are attenuated in virulence relative to low passage, virulent isolates by virtue of deletion of sequences shown to be present in low passage, virulent isolates but which are lacking in laboratory strains. The identification of these sequences was essential in order to prepare transfer vectors capable of shuttling deletions (or insertions such as selectable markers) resulting in an effective removal of coding information. Knowledge of the ORF usage on these DNAs permits deletion or insertion of one DNA into the other to specifically disrupt existing coding information. In addition, this invention identifies sequences which can be used as “spacer” DNA for substitution into deleted regions of HCXV clinical isolates for purposes of attenuation.

Cosmid subclones of Towne and Toledo

Cosmid subclones of the CMV(Towne) and CMV(Toledo) genomes were constructed according to the method of Kemble et al. (1996) J. Virol. 70: 2044, incorporated herein by reference. Human foreskin fibroblast (HF) cells were infected with either Towne or Toledo and following the development of extensive CPE, DNA was isolated from nucleocapsids by a procedure similar to that used for the preparation of HSV nucleocapsids (Denniston et al. (1981) Gene 15: 365, incorporated herein by reference). The DNA was partially digested with Sau3AI, fractionated by agarose t gel electrophoresis, and ligated to the BamHI site of BaI, XbaI digested arms of the SuperCos•A1 cosmid vector. SuperCos•A1 was derived from SuperCos-1 (Stratagene, San Diego, Calif.) by the insertion of an oligonucleotide incorporating SrfI and PacI recognition sequences flanking a unique BaHI site. The position of the cosmid subclones relative to the viral genome was identified by Southern and DNA sequence analyses.

Overlapping Cosmids for Virus Regeneration

Mapping the extent of the viral insert within the cosmid subclones was used as a basis to form specific Towne/Toledo chimeric viruses by choosing the appropriate cosmids from each virus. The ends of adjacent cosmids should overlap (˜200 bp or more) such that homologous recombination is permitted in eucaryotic cells.

To construct a Toledo based virus which lacked the Toledo U_(L)/b′ region and in its place contained the Towne U_(L)/b′ region, the following set of cosmids was used: Tol29, Tol58, Tol182, Tol22, Tol158, Tol24, Tn39, and Tn50. The resulting virus was designated Tol/Twn 39/50 (see FIG. 5). Other viruses were regenerated by cosmid cotransfection which lacked portions of the Toledo U_(L)/b′ region. Toledo based viruses were generated by the cotransfection of the Toledo cosmids, Tol29, Tol58, Tol182, Tol22, Tol158, Tol24, Tol 212, Tol187 OR Tol59, Tol150, Tol239, Tol235, Tol158, Tol24, Tol212, Tol187. Towne/Toledo chimeras lacking portions of the Toledo U_(L)/b′ region were regenerated by cotransfection of Tn43, Tn13, Tn24, Tn9, Tn42, Tn51, Tol212, Tol187. Because Tol 212 and Tol187 did not overlap, deletions resulted in the viruses regenerated from these cosmid sets which lacked varying portions of the Toledo U_(L)/b′ region.

Preparation of Cosmids for Cotransfection

A set of overlapping cosmid clones constituting the appropriate viral genome were individually digested with PacI to release the intact viral insert from the cosmid vector. The restriction enzyme was inactivated by heating at 65° C. for 20 minutes, the cosmids were combined and the DNA precipitated with ethanol. A CaPO₄ precipitate was formed from approximately 8 to 16 μg of this mixture and transfected using general transfection methods. The DNA was transfected into approximately 1×10⁶ low passage (<15 passes) HF, LF (human embryonic lung fibroblast) or IFIE1.3 (a gift of Ed Mocarski; these cells are immortalized HF cells that express the CMV major immediate early protein) cells. All these cells are permissive for CMV replication.

For HF and LF cells, approximately 1×10⁶ cells were plated onto a 25 cm² flask 3 to 5 hours prior to the addition of the DNA-CaPO₄ precipitate. At this point, the precipitate was adsorbed directly to the cell monolayer for 30 minutes prior to the addition of media. 2 ml of media was added and incubation continued for 4 hours at 37° C.

For IFIE1.3 cells, the cells were trypsinized approximately 16 hours prior to the addition of the DNA-CaPO₄ precipitate and seeded at a 1:2 density. At the appropriate time post seeding, the DNA-CaPO₄ precipitate was added in addition to 2 ml of media and incubated at 37° C. for 4 hours.

Following the 4 hour incubation, the DNA-CaPO₄ precipitate was removed, the cells incubated at 37° C. for 3 min in 15% glycerol in Hepes buffered saline, rinsed one time with media and fed with 5 ml of media. The media on the cells was changed every 3 to 4 days and plaques appeared in 10 to 21 days.

Construction of Recombinant CMV by Insertion of a gpt/LacZ Marker

Two plasmids encompassing the Toledo U_(L)/b′ region and derivatives thereof were constructed which contained a marker gene. A segment of DNA encompassing AD169 bases 156251-174483 was removed from pON2601 (Cha et al. (1996) J. Virol. 70: 78, incorporated herein by reference) and a PacI linker was introduced at AD169 base 174484 to yield a subclone of pON2601. FIGS. 3 and 4 show a schematic drawing of the open reading frames in the Toledo U_(L)/b′ region using sequence numbering from the Toledo U_(L)/b′ region DNA insert. A 4.8 kb DNA fragment containing the E. coli gpt and lac Z genes driven by the HSV thymindine kinase and β actin promoters (Prichard et al. (1996) J. Virol. 70: 3018, incorporated herein by reference), respectively, was then inserted into the NsiI site in Toledo UL148 within the pON2601 subclone. The resulting plasmid containing the gpt and lacZ insert in UL148 was designated pGD6. Toledo open reading frames UL138 to UL148 were removed from pGD6 by a BamHI collapse to produce the plasmid pGD7. Toledo recombinant viruses Tol pGD6 and Tol pGD7 were constructed using plasmids pGD6 and pGD7, respectively, as described (Prichard et al. (1996) op.cit).

Analysis of Recombinant CMV in SCID-hu (thy/liv) Mice

SCID-hu (thy/liv) mice were derived by implanting human fetal thymus and liver beneath the kidney capsule of a female C.B. −17 scid/scid IcrTac mouse (McCune et al. (1988) Science 241: 1632, incorporated herein by reference). The SCID-hu (thy/liv) mouse model serves as an animal model that can distinguish virulent from avirulent strains of CMV based on their replication levels within the human implant (Mocarski et al. (1993) Proc. Natl. Acad. Sci. (U.S.A.) 90: 104 and Brown et al. (1995) J. Infect. Dis. 171 1599, each incorporated herein by reference). Several weeks following implantation, the human implant on the murine kidney was surgically exposed and an inoculum of ˜10⁴ PFU of the appropriate virus was injected directly into the human tissue in a volume of 10-25 μl. The murine kidney/human implant was placed back into the animal in its natural position and the animal was recovered. 2 weeks following infection of the human tissue, the animal was sacrificed and the implant was removed and added to 2 ml of 4.5% skim milk/50% media.

The excised implant was homogenized with an automated Dounce apparatus (Glas-Col, Terre Haute, Ind.) and the suspension was stored at −80° C. until the titers were determined. The suspension was thawed at 37° C., sonicated on ice by three cycles of 10 sec on/10 sec off and centrifuged to remove the debris. The supernatent was recovered and the titer of CMV present was determined on confluent monolayers of HF cells. 7 to 10 days after plating the virus, the monolayers were fixed and stained with Giemsa and plaques enumerated.

FIG. 6 shows results from this experiment. The virulence of the Toledo strain CMV is attenuated by functional disruption of Toledo genome region open reading frames.

The difference in virulence between the Towne and Toledo strains appears to have resulted from genetic differences generated during the adaptation of Towne to growth in dipoid fibroblasts in culture. Both Towne and Toledo were originally isolated from the urine of a congenitally infected infant. Towne was subsequently passaged over 125 times in culture resulting in genetic alterations in the viral genome and an avirulent virus. The virulent Toledo virus, in contrast, was passaged approximately 5 times in diploid fibroblasts in order to produce material that could cause disease in humans.

These linked genetic and biological differences can be used to create a live, attenuated HCMV vaccine. The rationale for tissue culture adaptation of Towne was to generate a live, attenuated vaccine strain. Towne has been shown to be safe and somewhat immunogenic. Towne, however, is overattenuated. The immune response induced by inoculation with Towne does not protect against subsequent HCMV infection as effectively as that generated by natural infection. Vaccine candidates can be generated by replacing genetic elements of the overattenuated Towne strain with homologous portions of the virulent Toledo strain. Through the analysis of these “chimeric” viruses, a skilled artisan can select those that have the level of desirable characteristics of Towne and be attenuated to a more efficacious degree.

Our first four chimeric viruses, as a set, will contain the entire Toledo genome introduced into the Towne genetic background. Each individual chimera of the set will contain approximately 40-55% of the Toledo genome; the remainder will be derived from Towne. Each of these chimeras will contain the U_(L)/b′ region derived from Toledo. Genes within this region of the Toledo genome can affect cell tropism of HCMV. The viruses, designated chimera I, II, III, and IV were constructed from the cosmids listed in Table 1 (see also FIGS. 9 and 10).

TABLE 1 Cosmids used to generate specific chimeras. Viruses I II III IV Tn46a Tn46 Tn46 Tol29 Tol58b Tn45 Tn45 Tol58 Tol182 Tol239 Tn23 Tn23 Tn47 Tol22 Tn47 Tn47 Tn44 Tol158 Tol184 Tn44 Tn26 Tn26 Tol24 Tn26 Tn20 Tn20 Tol212 Tol212 Tol11 Tol11 Tol11 Tol122

Large quantities of each cosmid were prepared by purification of the E. coli produced material over a Qiagen column as described by the manufacturer. 10 micrograms of each cosmid was digested with the restriction enzyme Pac I (New England Biolabs) to physically separate cosmid vector from viral sequences. Following digestion, the enzyme was inactivated by incubation at 65oC for 20 minutes and the appropriate cosmids were combined, precipitated with ethanol in the presence of 0.3M sodium acetate, rinsed in 70% ethanol, and air-dried briefly. The resulting DNA was solubilized in approximately 100 microliters of 10 mM Tris pH 7.5/1 mM EDTA. Various amounts of the cosmid mix was transfected by the calcium phosphate technique into permissive fibroblast cells, specifically human lung fibroblasts (LF, prepared in our laboratory), human neonatal foreskin fibroblast (HF, a gift of Dr. Ed Mocarski, Stanford University), MRC-5 (ATCC) or IFIE1.3 cells (a gift of Ed Mocarski, Stanford University). The IFIE1.3 cell line constitutively expresses the HCMV ie1 gene product and has been transformed with the human papilloma virus E6 and E7 genes transduced by a retrovirus vector. 3 to 5 hours after transfection the cells were shocked by incubation in 15% glycerol/Hepes buffered saline for 3 minutes at 37oC and fed with DME/10% fetal bovine serum. 7 to 10 days after transfection plaques with distinct HCMV CPE were evident.

Plaques derived from the chimeras were allowed to grow until 100% of the monolayer exhibited CPE. At this point, DNA was extracted from the supernatent and cellular fractions and analyzed by restriction enzyme digestion. The structures of the viruses can be deduced by the cosmids used for construction of the chimera and confirmed by comparing the EcoRI digestion pattern to the maps derived for Towne and Toledo (see FIG. 10). Table 2 describes the composition of each of the chimeras, the nucleotide limits are derived from sequence analysis of the end of each cosmid insert and its homology to the AD169 strain of HCMV, which has been sequenced in its entirety. All of the chimeras had restriction enzyme patterns consistent with the proposed structures.

TABLE 2 Genetic composition of the chimeras. Chime- ra Towne DNA Toledo DNA Crossover Region I   1–3799 15750–67568  3800–15749  81647–170499 175069–203136 67569–81646 205803 to S term. 170500–175068 II   1–47985  53244–~110000 47986–53243  138834–170499 175069–203136 ~110000–130833  205803 to S term. 170500–175068 203137–205802 III    1–~99000* 108094–203136 ~99000–108093 205803 to S term 203137–205802 IV  43981–145583   1–41356 41357–43980 150754–S term 145584–150753 *Sequence at end of cosmid was undefinable. Nucleotide number is not exact crossover region is the region of cosmid overlap. The contribution ofeach virus to this region has yet to be defined.

Two other chimeras are constructed based on an observation derived from the sequence analysis of several different members of the beta-herpesvirus family including HCMV, human herpesvirus 6 (the causative agent of roseola) and murine cytomegalovirus. Representative members of each of these viruses have been sequenced in their entirety and a “core” set of genes corresponding to HCMV UL23 to UL122 are conserved among these evolutionarily divergent entities (Chee, et al. 1990; Gompels, et al. 1995; Rawlinson, et al. 1996, incorporated herein by reference). These core genes contribute to DNA replication, virion structure, and other basic features of the virus. Genes outside this core region are involved in virus-host and virus-immune system interaction and may determine specific properties of virus biology. Replication of the Chimeras was tested in SCID-hu mice having a thy/liv sandwich under the kidney capsule; representative data is shown in FIG. 11.

Two additional chimeras are constructed: one which has the core derived from Toledo with the remainder of the genes derived from Towne and an inverse construct in which the core is derived from Towne and the remainder of the genes are from Toledo. These viruses are constructed through the use of overlapping cosmids and derivatives of the cosmids. Table 3 outlines the constructs that can be used to generate these two chimeric viruses.

TABLE 3 Construction of chimeras containing conserved core regions. Towne Core/Tol Noncore Tol Core/Towne Noncore Tol 29 Tn43 Tol58 nts: 3800–27862 Tn45 nts: 7854–27862 Tn45 nts: 27500–53243 Tol 58: 27500–43980 Tn23 Tol 182 Tn47 Tol 22 Tn44 Tol 158 Tn26 Tol 24 Tn20 Tol212 nts: 145584–17200 Tol11 nts: 170852–188890 Tn 39 nts: 170852–183512 Tol122 Tn 15

All of these viruses are used to inoculate healthy adult human volunteers. These individuals are assessed for symptoms of HCMV disease, including fever, malaise, and abnormal liver enzyme levels. Hallmarks of viral infection are also assessed by measuring HCMV specific antibody titers before and after inoculation as well as viral culture for the isolation of infectious virus from bodily fluids. A successful vaccine candidate is identified as a strain that maintains the safety profile of Towne while stimulating a greater immune response to the virus.

FIG. 12 shows schematic depiction of the Toledo genome incomparison with highly passaged Towne (short genome) and low-passage Towne (long genome).

The foregoing description of the preferred embodiments of the present invention has been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise form disclosed, and many modifications and variations are possible in light of the above teaching.

Such modifications and variations which may be apparent to a person skilled in the art are intended to be within the scope of this invention. 

1-12. (canceled)
 13. A chimeric cytomegalovirus (CMV) virus which comprises: (a) a polynucleotide sequence of a high-passage Towne genome from nucleotides 1 to 47985; (b) a polynucleotide sequence of a Toledo genome from nucleotides 53244 to about 110000; (c) a polynucleotide sequence of a high-passage Towne genome from nucleotides 138834 to 170499; (d) a polynucleotide sequence of a Toledo genome from nucleotides 175069 to 203136; and (e) a polynucleotide sequence of a high passage Towne genome from nucleotides 205803 to S-term, wherein the high-passage Towne genome has been passaged at least 50 times and wherein the nucleotide number of the Towne and Toledo genomes is according to the numbering convention of the AD 169 genome.
 14. A chimeric CMV virus which comprises: (a) a polynucleotide sequence of a high-passage Towne genome from nucleotides 1 to 47985; (b) a crossover region comprising SEQ ID NO.: 27; (c) a polynucleotide sequence of a Toledo genome from nucleotides 53244 to 110000; (d) a crossover region comprising SEQ ID NO.: 28; (e) a polynucleotide sequence of a high-passage Towne genome from nucleotides 138834 to 170499; (f) a crossover region comprising SEQ ID NO.: 25; (g) a polynucleotide sequence of a Toledo genome from nucleotides 175069 to 203136; (h) a crossover region comprising SEQ ID NO.: 26 and (i) a polynucleotide sequence of a high passage Towne genome from nucleotides 205803 to S-term, wherein the high-passage Towne genome has been passaged at least 50 times and wherein the nucleotide number of the Towne and Toledo genomes is according to the numbering convention of the AD169 genome.
 15. An immunogenic composition comprising the chimeric virus of claim
 13. 16. An immunogenic composition comprising the chimeric virus of claim
 14. 17. A method of inducing an immune response comprising administering to a human an immunological composition of claim 15 in an amount sufficient to stimulate an immune response in said human.
 18. A method of inducing an immune response comprising administering to a human an immunological composition of claim 16 in an amount sufficient to stimulate an immune response in said human. 